Semiconducting organic materials are attracting a great deal of interest due to their processability and the broad spectrum of optical and electronic properties that may be selected according to the structure of the organic material.
One application of such materials is in switching devices, in particular as organic field effect transistors as described in, for example, Adv. Mater. 1998 10(5), 365-377.
Another application is in opto-electrical devices using a semiconducting organic material for light emission (an organic light emitting device or “OLED”) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic electroluminescent device, electrons and holes are injected into a layer of electroluminescent semiconducting material where they combine to generate excitons that undergo radiative decay. Holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the electroluminescent material; electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material. In WO 90/13148 the organic light-emissive material is a polymer, namely poly(p-phenylenevinylene) (“PPV”). This class of device is commonly known as a polymer light emitting device (PLED). In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”).
One alternative to PPVs are 2,7-linked polyfluorenes as disclosed in EP 0842208 which have attracted attention because of their advantage of solution processability, such as suitability for inkjet printing. Furthermore, fluorene monomers with appropriate leaving groups are amenable to Suzuki or Yamamoto polymerisation. Suzuki polymerisation in particular affords a great deal of control over the regioregularity and therefore the properties of the polymer. Fluorene repeat units may therefore be used as a “building block” in creating co-polymers with a wide range of charge transporting and/or emissive properties.
However, there are a number of disadvantages associated with polyfluorenes which have led to a search for alternative electron transporting and light emitting units. These disadvantages include the tendency of polyfluorenes to aggregate and the fact that when blue light emission occurs from fluorene based polymers the emission does not occur in the region of the electromagnetic spectrum in which the human eye is most sensitive.
One alternative to fluorene repeat units are trans-indenofluorene repeat units (illustrated below) as disclosed in, for example, Macromolecules 2000, 33(6), 2016-2020 and Advanced Materials, 2001, 13, 1096-1099.
Polymers comprising the tetraoctyl trans-indenofluorene unit are described as having
a bathochromically shifted emission wavelength which leads to a blue emission colour matched to the sensitivity of the human eye. However, poly(trans-indenofluorenes) have a lower conductivity than corresponding polyfluorenes.
It is therefore an object of the invention to provide a repeat unit that possesses the advantages of trans-indenofluorene over fluorene without suffering from loss of conduction.